1. Field of the Invention
The present invention relates to a wavelength-variable semiconductor laser, which is used in the fields of optical communication and optical information processing (e.g., optical disks, displays, and the like); an optical integrated device structured by such a wavelength-variable semiconductor laser and an optical waveguide device; and a method for producing the same. More particularly, the present intention relates to the structure of an integrated short-wavelength light source structured by a semiconductor laser chip, including the above-mentioned wavelength-variable semiconductor laser, and an optical waveguide type wavelength converting device; and a method for producing the same.
2. Description of the Related Art
In the field of optical communication, the development of a small and low-cost optical module in which a semiconductor laser, an electronic element, an optical fiber, and the like are hybrid-integrated on a quartz type light wave circuit platform has been highly valued.
The important factor to be considered in connection the above is to fix each element with high positional accuracy so as to minimize transfer loss as much as possible. For such a purpose, a surface mounting type optical module which directly couples a semiconductor laser with a single-mode fiber using a Si substrate with a V-shaped groove has been realized (e.g., Proceedings of the 1997 IEICE General Conference, C-3-63).
According to such a technique, markers are formed on a Si substrate and a semiconductor laser element, and a center of the V-shaped groove and a light-emitting point of the semiconductor laser element are detected by the image recognition of the markers so as to perform positioning adjustment (positional alignment) with high accuracy. With such a structure, mounting deviation can be suppressed to about .+-.0.61 .mu.m in an x-direction, and to about .+-.1 .mu.m in a z-direction.
In the field of optical information processing, a small short-wavelength light source is demanded in order to realize a higher-density optical disk and a high definition display. Techniques for realizing a short-wavelength includes a second harmonic-wave generation (hereinafter, referred to as "SHG") method which uses a semiconductor laser and an optical waveguide type wavelength converting device employing a quasi-phase-matching (hereinafter, referred to as "QPM") method (for example, see Yamamoto et al., Optics Letters, Vol. 16, No. 15, p. 1156, 1991).
FIG. 1 is a view showing a general structure of a blue light source using an optical waveguide type wavelength converting device.
According to the structure shown in FIG. 1, a wavelength-variable semiconductor laser 110 having a distributed Bragg reflection (hereinafter, referred to as "DBR") region is used as a semiconductor laser 110. Hereinafter, the wavelength-variable semiconductor laser having the DBR region is referred to as a "DBR semiconductor laser" or a "DBR laser".
The DBR semiconductor laser 110 is, for example, a 100 mW class AlGaAs type DBR semiconductor laser of a 0.85 .mu.m band. The DBR semiconductor laser 110 includes an active layer region 112 and a DBR region 111. By varying a current injected into the DBR region 111, it is possible to vary an oscillation wavelength.
On the other hand, an optical waveguide type wavelength converting device 116, which serves as a wavelength converting element, includes an optical waveguide 115 formed in a X-cut MgO-doped LiNbO.sub.3 substrate 113 and periodic domain-inverted regions 114. The semiconductor laser 110 and the wavelength converting device 116 are fixed onto submounts 117 and 118, respectively, in a junction-up manner.
Laser beams obtained from an outputting end surface of the DBR semiconductor laser 110 are directly coupled with the optical waveguide 115 of the optical waveguide type wavelength converting device 116. Specifically, by adjusting the positional relationship between the DBR semiconductor laser 110 and the optical waveguide type wavelength converting device 116 on the submounts 117 and 118, laser beams of about 60 mW are coupled with the optical waveguide 115 of the wavelength converting device 116 for the laser output of about 100 mW from the semiconductor laser 110. Moreover, by controlling an amount of the current injected into the DBR region 111 of the DBR semiconductor laser 110, the oscillation wavelength thereof is fixed within the allowable range of the phase matching wavelength in the optical waveguide type wavelength converting device 116. With such a structure, at present, it is possible to obtain blue light with a wavelength of 425 nm at the output of about 10 mW.
FIG. 2 is a view showing a general structure of a blue light source using a domain-inverted type waveguide device.
A DBR semiconductor laser 221 (i.e., a 100 mW class AlGaAs type DBR semiconductor laser of a 0.85 .mu.m band) includes a DBR region 228 for fixing an oscillation wavelength. Within the DBR region 228, an internal heater (not shown) is provided so as to vary an oscillation wavelength. On the other hand, a domain-inverted type waveguide device 224 serving as a wavelength converting element includes an optical waveguide 226 formed in a X-cut MgO-doped LiNbO.sub.3 substrate 225 and periodic domain-inverted regions 227.
A laser beam 229, which is output from the semiconductor laser 221 and is collimated by a collimator lens 222 (numerical aperture NA=0.5), is converged onto an end surface of the optical waveguide 226 in the domain-inverted type waveguide device 224 by a focusing lens 223 (NA=0.5). The laser beam 229 is then coupled with the optical waveguide 226 having the domain-inverted regions 227. Specifically, for a laser output of about 100 mW, it is possible to allow a laser beam of about 70 mW to be coupled with the optical waveguide 226. With such a structure, the oscillation wavelength thereof is fixed within the allowable range of the phase matching wavelength in the domain-inverted type waveguide device 224 by controlling an amount of the current injected into the DBR region 228 of the DBR semiconductor laser 221.
The blue light output thereby obtained increases in proportional to the square value of the output of the laser beam coupled with the optical waveguide 226. Therefore, coupling efficiency is a critical factor in order to obtain blue light with a high output.
In the case of the surface mounting type optical module which directly couples a semiconductor laser with a single-mode fiber, in order to couple a semiconductor laser beam into the optical fiber at a high efficiency, it is necessary to have positional adjustment (alignment) accuracy in the order of a submicron with respect to an x-direction and a y-direction which are parallel to the cross-section of the optical fiber and in the order of several microns with respect to a z-direction along the optical axis of the optical fiber. In such a case, the optical fiber is typically fixed with high accuracy by using a V-shaped groove. In addition, the semiconductor laser chip has been conventionally positioned with high accuracy with respect to an x-direction, which is parallel to the surface of the mounting substrate within the cross-section of the optical fiber, and a z-direction along the optical axis of the optical fiber. However, as to the positioning of the semiconductor laser chip with respect to a y-direction, which is parallel to the surface of the mounting substrate within the cross-section of the optical fiber, it is generally difficult to perform positioning (alignment) adjustment and fixation with the accuracy of a submicron or less due to the existence of a solder layer for fixation.
On the other hand, in the short-wavelength light source structured by a semiconductor laser and an optical waveguide type wavelength converting device (i.e., an optical waveguide device), the lens coupling type module as shown in FIG. 2 has been realized. In order to apply such a short-wavelength light source for commercial use, miniaturization of the light source and realization of low cost are essential. According to the present lens coupling type module, however, it is necessary to perform the adjustment of the lens and the semiconductor laser, resulting in low productivity. Moreover, the miniaturization of the overall size is limited to about several cc at most. Although there exists a direct coupling technique as a method for realizing further miniaturization, in order to realize a further reduction in cost of the directly coupling type module, a modularizing technique capable of realizing a more improved productivity and capable of positioning with higher accuracy is required.
The spread angle of a semiconductor laser is generally small in the direction parallel to the surface of the mounting substrate (i.e., x-direction), and large in the direction perpendicular to the surface of the substrate (i.e., y-direction). The spread angle in the optical waveguide produced by a proton exchange method is also small in the direction parallel to the surface of the mounting substrate (i.e., x-direction), and large in the direction perpendicular to the surface of the substrate (i.e., y-direction). Thus, more accurate positioning adjustment (alignment) is required in the direction perpendicular to the substrate surface (i.e., y-direction), i.e., requiring the adjustment accuracy of a submicron or less. In other words, it is necessary to control the height from the submount, onto which the semiconductor laser is mounted, to the active layer with high accuracy. However, the thickness of a GaAs substrate generally used as a substrate in an AlGaAs semiconductor laser is not typically controlled with the accuracy of a submicron level. On the other hand, the thicknesses of epitaxial layers, such as an active layer, and that of an electrode are controlled with high accuracy. Therefore, in order to control a position in a height direction (i.e, the y-direction) with high accuracy, the semiconductor laser needs to be mounted in a junction-down manner (i.e, mounted in a direction such that the surface on which the active layer is provided, i.e., the epitaxial growth surface, contacts the submount).
Moreover, with regards to a semiconductor laser mounted in a junction-up manner, the heat transfer condition in the active layer region is not satisfactory, and consequently, sufficient reliability of the laser can not be obtained. From such a standpoint, it is necessary to mount the semiconductor laser in a junction-down manner.
The DBR semiconductor laser as a wavelength-variable semiconductor laser, constituting an optical waveguide integrated light source, varies an oscillation wavelength thereof by controlling an amount of current injected into the DBR region. Specifically, by changing a temperature in the DBR region by the current injection so as to change a refractive index, the wavelength of light fedback from the DBR region is varied. According to such a principle, since the heat transfer condition in the DBR region is improved upon performing the junction-down mounting, it becomes rather difficult to realize a satisfactory control for varying an oscillation wavelength.
For fixing the optical waveguide device, it is impossible to use a structure such as a V-shaped groove which is typically used for fixing an optical fiber. Thus, especially in the y-direction, highly accurate positioning (alignment) adjustment is required.